1. Field of the Invention
This invention relates to the field of apparatus and methods for detecting and quantifying light emissions, and more particularly, to detecting and quantifying light emitted from luminescent-based assays. Still more particularly, this invention pertains to apparatus and methods for detecting and quantifying luminescence such as bioluminescence and/or chemiluminescence from luminescent assays as an indicator of the presence or amount of a target compound. Preferred embodiments of the invention include as an imaging device a charge coupled device (CCD) camera and a computer for analyzing data collected by the imaging device. Further preferred embodiments include the capacity for use in high throughput screening (HTS) applications, and provide for robot handling of assay plates.
2. Description of the Related Art
The analysis of the luminescence of a substance, and specifically the analysis of either bioluminescence (BL) or chemiluminescence (CL), is becoming an increasingly useful method of making quantitative determinations of a variety of luminescent analytes.
Recently, methods have been introduced that utilize luminescence detection for quantitatively analyzing analytes in an immunoassay protocol. Such luminescence immunoassays (LIA) offer the potential of combining the reaction specificity of immunospecific antibodies or hybridizing nucleic acid sequences and similar specific ligands with the high sensitivity available through light detection. Traditionally, radioactive reagents have been used for such purposes, and the specificity and sensitivity of LIA reagents is generally comparable to those employing traditional radiolabelling. However, LIA is the preferred analytical method for many applications, owing to the nontoxic nature of LIA reagents and the longer shelf lives of LIA reagents relative to radioactive reagents.
Among other luminescent reagents, chemiluminescent compounds such as 1,2-dioxetanes, developed by Tropix, Inc. and other stable chemiluminescent molecules, such as xanthan esters and the like, are in commercial use. These compounds are triggered to release light through decomposition triggered by an agent, frequently an enzyme such as alkaline phosphatase, which is present only in the presence, or specific absence, of the target compound. The detection of light emission is a qualitative indication, and the amount of light emitted can be quantified as an indicator of the amount of triggering agent, and therefore target compound, present. Other well known luminescent compounds can be used as well.
Luminescent release may sometimes be enhanced by the presence of an enhancement agent that amplifies or increases the amount of light released. This can be achieved by using agents which sequester the luminescent reagents in a microenvironment which reduces suppression of light emission. Much biological work is done, perforce, in aqueous media. Water typically suppresses light emission. By providing compounds, such as water soluble polymeric onium salts (ammonium, phosphonium, sulfonium, etc.) small regions where water is excluded that may sequester the light emitting compound may be provided.
The majority of instrumentation used to monitor light emitting reactions (luminometers) use one or more photomultiplier tubes (PMTs) to detect the photons emitted. These are designed to detect light at the low light levels associated with luminescent reactions. The rate at which a PMT based microplate luminometer can measure signal from all wells of the plate is limited by the number of PMTs used. Most microplate luminometers have only one PMT so a 384 well plate requires four times longer than is required to read a 96-well plate.
The nature of biological research dictates that numerous samples be assayed concurrently, e.g., for reaction of a chemiluminescent substrate with an enzyme. This is particularly true in gene screening and drug discovery, where thousands of samples varying by concentration, composition, media, etc. must be tested. This requires that multiple samples be reacted simultaneously, and screened for luminescence. However, there is a need for high speed processing, as the chemiluminescence or bioluminescence may diminish with time. Simultaneously screening multiple samples results in improved data collection times, which subsequently permits faster data analysis, and contingent improved reliability of the analyzed data.
In order for each specific sample analyte's luminescence to be analyzed with the desired degree of accuracy, the light emission from each sample must be isolated from the samples being analyzed concurrently. In such circumstances, stray light from external light sources or adjacent samples, even when those light levels are low, can be problematic. Conventional assays, particularly those employing high throughput screening (HTS) use microplates, plastic trays provided with multiple wells, as separate reaction chambers to accommodate the many samples to be tested. Plates currently in use include 96- and 384-well plates. In response to the increasing demand for HTS speed and miniaturization, plates having 1,536 wells are being introduced. An especially difficult impediment to accurate luminescence analysis is the inadvertent detection of light in sample wells adjacent to wells with high signal intensity. This phenomenon of light measurement interference by adjacent samples is termed ‘crosstalk’ and can lead to assignment of erroneous values to samples in the adjacent wells if the signal in those wells is actually weak.
Some previously proposed luminometers include those described in U.S. Pat. Nos. 4,772,453; 4,366,118; and European Patent No. EP 0025350. U.S. Pat. No. 4,772,453 describes a luminometer having a fixed photodetector positioned above a platform carrying a plurality of sample cells. Each cell is positioned in turn under an aperture through which light from the sample is directed to the photodetector. U.S. Pat. No. 4,366,118 describes a luminometer in which light emitted from a linear array of samples is detected laterally instead of above the sample. Finally, EP 0025350 describes a luminometer in which light emitted through the bottom of a sample well is detected by a movable photodetector array positioned underneath the wells.
Further refinements of luminometers have been proposed in which a liquid injection system for initiating the luminescence reaction just prior to detection is employed, as disclosed in EP 0025350. Also, a temperature control mechanism has been proposed for use in a luminometer in U.S. Pat. No. 4,099,920. Control of the temperature of luminescent samples may be important, for example, when it is desired to incubate the samples at an elevated temperature.
A variety of light detection systems for HTS applications are available in the market. These include the LEADseeker™ from Amersham/Pharmacia, the ViewLux™ offered by PerkinElmer and CLIPR™ from Molecular Devices. These devices are all expensive, large dimensioned (floorbased models), exhibit only limited compatibility with robotic devices for plate preparation and loading, have a limited dynamic range, and/or use optical detection methods which do not reduce, or account for, crosstalk. The optical systems used are typically complex teleconcentric glass lens systems, which may provide a distorted view of wells at the edges of the plates, and the systems are frequently expensive, costing in excess of $200,000.00. Perhaps the most popular detection apparatus is the TopCount™, a PMT-based detection system from Packard. Although the TopCount™ device has a desirable dynamic range, it is not capable of reading 1,536 well plates, and it does not image the whole plate simultaneously.
Crosstalk from adjacent samples remains a significant obstacle to the development of improved luminescence analysis in imaging-based systems. This can be appreciated as a phenomenon of simple optics, where luminescent samples produce stray light which can interfere with the light from adjacent samples. Furthermore, the development of luminometers capable of detecting and analyzing samples with extremely low light levels are particularly vulnerable to crosstalk interference.